THE EXO-SKELETON 11 



the point of view of technique. Rather less than half its total weight 

 consists of the substance known as chit in, the peculiarly resistant 

 properties of which are a formidable obstacle in the manipulation of 

 insect tissues. Chitin is insoluble in acids and alkalis, in alcohol and 

 ether, and in fact in all the ordinary laboratory reagents. It is also 

 extremely impermeable, and unless apertures in the integument are made 

 reagents will not penetrate to the soft tissues. It is pigmented, and when 

 free from cellular material is more or less transparent. Its colour can 

 be readily removed by bleaching agents, such as chlorine gas. At the 

 moment of emergence from the pupal covering the chitin of the imago is 

 soft and colourless, and it is only after exposure to air and light that 

 it becomes pigmented and rigid. When in thin layers, with much cellu- 

 lar material, it forms a tough membrane, but when in thick sheets it has 

 only a small amount of resilience. It resists the action of heat, but 

 is made harder and more brittle by even such moderate temperatures as 

 are used in a paraffin bath. Prolonged immersion in alcohol has the 

 same effect. 



The disposition of the chitin in the integument is as follows. (Plate 

 I, fig. 5.) On the internal surface of the exo-skeleton there is a 

 single layer of cells set on a thin basement membrane. These are the 

 chitinogenous cells, and external to them there are found many layers of 

 solid and structureless chitin, which has been secreted by the cells 

 in very much the same manner as that in which the nails of vertebrates 

 are produced. The basement membrane and the chitinogenous cells 

 form a continuous layer throughout the body of the insect, and there 

 is always a layer of chitinous tissue external to them, the thickness 

 of which is varied in accordance with the structural requirements. 

 In such parts as the thorax, where strength is necessary on account 

 of the great development of the wing muscles, the layer of chitin is 

 very thick, while in the region of joints, where flexibility is required, 

 it is thin. But it is important to remember that even in thin sheets 

 chitin is quite impermeable to any of the ordinary fixatives. 



The hairs and scales which are present on the integument, and which 

 are so much used in classification, are outgrowths from the exo-skeleton. 

 They are of two kinds, sensory hairs and others the function of which is 

 unknown. The sensory hairs, such as those found at the distal end of 

 the proboscis, on the palps, and on the antennae in some forms, are 

 hollow, and contain delicate filaments of nervous tissue, enclosed in a 

 central canal. They arise from single cells of the chitinogenous layer, 

 the base of each hair piercing the chitinous laminae, and emerging from 



